Any gas taken to depth in a scuba tank will be unaffected as long as it remains in
the tank. Once it leaves the tank and enters the diver's lungs it will have the same
pressure as the surrounding water, i.e., the ambient pressure. This statement is true for
the two major components of compressed air (nitrogen and oxygen), as well as for any
gaseous impurities (e.g., carbon monoxide).

WHAT IS NITROGEN NARCOSIS?

Nitrogen narcosis, also called "rapture of the deep" and "the
martini effect," results from a direct toxic effect of high nitrogen pressure on
nerve conduction. It is an alcohol-like effect, a feeling often compared to drinking a
martini on an empty stomach: slightly giddy, woozy, a little off balance.

Nitrogen narcosis is a highly variable sensation but always depth-related. Some divers
experience no narcotic effect at depths up to 130 fsw, whereas others feel some effect at
around 80 fsw. One thing is certain: once begun, the narcotic effect increases with
increasing depth. Each additional 50 feet depth is said to feel like having another
martini. The diver may feel and act totally drunk. Underwater, of course, this sensation
can be deadly. Divers suffering nitrogen narcosis have been observed taking the regulator
out of their mouth and handing it to a fish!

In The Silent World, Cousteau wrote about his early experiences with the aqua
lung:

I am personally quite receptive to nitrogen rapture. I like it and fear it like doom.
It destroys the instinct of life. Tough individuals are not
overcome as soon as neurasthenic persons like me, but they have
difficulty extricating themselves. Intellectuals get drunk early and suffer
acute attacks on all the senses, which demand hard fighting to
overcome. When they have beaten the foe, they recover quickly. The
agreeable glow of depth rapture resembles the giggle-party jags of the
nineteen-twenties when flappers and sheiks convened to sniff nitrogen protoxide.

L'ivresse des grandes profoundeurs has one salient advantage over
alcohol no hangover. If one is able to escape from its zone, the brain
clears instantly and there are no horrors in the morning. I cannot read
accounts of a record dive without wanting to ask the champion how drunk he was.

The effect, thought due to a slowing of nerve impulses from inert gas under high
pressure, is not unique to nitrogen; it can occur from many gases (though not helium). The
effect is similar to what patients experience inhaling an anesthetic such as nitrous oxide
(N2O). With increasing pressure of inhaled N2O
there is a progression of symptoms, from an initial feeling of euphoria to drunkenness and
finally to unconsciousness.

Every year there are diving deaths attributed to nitrogen narcosis, mainly among divers
who exceed recreational depth limits. To prevent the problem commercial divers switch to a
mixture of helium and oxygen (heliox) at depths exceeding around 170 fsw. Helium is much
less soluble in tissues than nitrogen, and therefore is less likely to impair behavior
(divers using helium still have to decompress to prevent DCS). Even setting aside the
added cost and complexity, helium offers no advantage for recreational divers over
ordinary air.

Because of similar (and additive) effects to excess nitrogen, alcohol should be avoided
before any dive. A reasonable recommendation is total abstinence at least 24 hours before
diving; by that time effects of alcohol should be gone.

Unlike the effects of alcohol, nitrogen narcosis dissipates quickly, as soon as the
diver ascends to a safe level (usually less than 60 feet depth). There is also some
evidence that some divers can become partially acclimated to the effects of excess
nitrogen; the more frequently they dive the less each subsequent dive appears to affect
them.

WHAT IS OXYGEN TOXICITY, AND CAN IT DEVELOP WHILE DIVING?

Oxygen toxicity is any injury or discomfort to the body from inhaling too much oxygen (see box). High concentrations of oxygen delivered at
atmospheric pressure can harm the lungs. When diving, any given concentration of oxygen
comes under higher pressure than atmospheric, thus increasing the amount inhaled and the
potential for toxicity. Above atmospheric pressures, oxygen can also affect the central
nervous system, and cause seizures and convulsions. Thus oxygen toxicity is a major
potential hazard in some diving but not, as it turns out, recreational diving.

Oxygen is a vital gas, the absence of which leads to death in a few minutes. People
with healthy lungs only need the amount of oxygen in the atmosphere, no more or less.
Anything more than 21% oxygen is considered "supplemental oxygen."

Supplemental oxygen, like any drug, can be toxic at high doses; since oxygen is a gas
the "dose" is based on both the percentage of oxygen inhaled and the ambient
pressure. Patients who are ill from low blood oxygen receive a higher than normal
percentage of oxygen as treatment (i.e., greater than 21%). Scuba divers, because of the
increase in gas pressures with depth, inhale a higher than normal oxygen pressure;
the percentage is the same, since compressed air is still 21% oxygen at any depth.

However, since pressure increases with depth, the deeper one dives the higher
the total pressure of oxygen that is inhaled. Too high an inhaled oxygen pressure can be
toxic to the lungs and central nervous system. Oxygen toxicity is the reason why very deep
diving (e.g., greater than about 170 fsw) is safely accomplished not with compressed air,
which contains 21% oxygen, but with a gas mixture that has a much lower percentage, e.g.,
10% O2. Such a low oxygen percentage would be dangerous at sea
level, but at great depth, due to the high ambient pressure, it is more than adequate to
sustain life.

Recreational scuba divers adhering to the dive tables have no
significant risk of oxygen toxicity. At 35 feet depth, where RSD tables allow the diver to
spend well over two hours on a non-repetitive dive, the PAO2 (oxygen
pressure in the lungs) is the same as from breathing 43% oxygen at sea level, i.e.,
non-toxic. At the maximum RSD depth of 130 feet, the PAO2 from
breathing compressed air is about the same as from breathing 100% oxygen at sea level.
This level of oxygen would only begin to cause trouble if inhaled for at least an hour.
The few minutes of bottom time that the tables allow at 130 fsw is simply not long enough
to pose a significant risk from oxygen toxicity.

WHAT EXACTLY DETERMINES RISK OF OXYGEN TOXICITY?

The occurrence and type of oxygen toxicity correlate with the O2 concentration,
the ambient pressure, the length of time supplemental O2 is inhaled,
and the diver's level of activity.

Range of oxygen concentrations. The concentration of inspired oxygen can
vary from zero to 100% (the maximum). The concentration in ordinary air is 21% (whether
compressed or not, and regardless of the depth at which it is inhaled). The higher the
concentration of O2 the greater the risk of oxygen toxicity.

Range of ambient pressures. Ambient pressure can range from zero (outer
space), to one atmosphere (sea level), to several atmospheres (in a hyperbaric chamber or
under water). On land, outside of a chamber, oxygen can be administered only at the
surrounding atmospheric pressure, which can vary from 1 atmosphere (sea level) to about
.33 atmosphere (summit of Mt. Everest). The higher the ambient pressure, the greater
the risk of oxygen toxicity.

Length of time oxygen is inhaled. Supplemental oxygen can be given
anywhere from a few seconds to lifelong. How long O2 is given
depends on the condition being treated, the concentration used, and the ambient pressure. The
longer supplemental oxygen is inhaled, the greater the risk of oxygen toxicity.

Level of activity. This is the least quantifiable aspect of oxygen
toxicity. Once the threshold of oxygen toxicity is reached (based on atmospheres of O2), the more active the diver the greater the risk of developing actual
toxicity.

Since air contains 21% oxygen, the amount of oxygen inhaled at sea level is .21 atm. O2; this amount is safe to breathe forever. From clinical experience it
appears that patients can breathe .40 atm O2 indefinitely, and
possibly up to .60 atm O2 for weeks at a time (equivalent to 40% O2 and 60% O2 at sea level, respectively), without
apparent oxygen toxicity.

In healthy subjects, 100% oxygen at atmospheric pressure (1 atm. O2)
causes chest discomfort, pain and cough after only a few hours. If inhaled continuously
over 24 hours, 1 atm. O2 can lead to lung congestion (pulmonary
edema) and, if continued, death. Obviously, doctors try not to use high concentrations of
oxygen unless absolutely necessary. Patients who require 100% oxygen because of heart or
lung disease are critically ill and will almost always be cared for in a hospital
intensive care unit.

The most serious potential harm from inhaling supplemental oxygen at sea level pressure
is lung injury, which develops slowly, over many hours. At depth the most serious harm
from too much oxygen is a seizure, which can occur in just a few minutes of oxygen
breathing.

HOW DO ATMOSPHERES OF O2 RELATE TO OXYGEN TOXICITY?

Although potentially toxic, 1 atm. O2 does not cause seizures.
However, when 100% oxygen is delivered at pressures two or more times sea level pressure,
the first toxic manifestation can be a seizure. A seizure is a sudden electrical discharge
from the brain that causes uncontrolled muscle movement. If seizures occur under water the
diver will likely be unable to breathe through the regulator and will drown (if rescue is
not immediate).

Atmospheres of O2 is the major determinant of oxygen toxicity;
the risk increases directly with the atmospheres of oxygen inhaled. A diver breathing
compressed air (21% oxygen) at 4.76 atmospheres (124 fsw) has the same risk of developing
oxygen toxicity as when breathing 100% O2 at sea level (assuming the
same level of activity). In either situation the diver is breathing one atmosphere of
oxygen (1 atm. O2).

Exposure to high oxygen pressures at RSD depths is not long enough to cause oxygen
toxicity. Oxygen toxicity is mainly a concern for the deep diver, for divers breathing
mixtures that contain more than 21% O2 (e.g., Nitrox), and for
patients undergoing hyperbaric oxygen therapy. The thresh-old beyond which oxygen toxicity
is a major concern is about 1.3-1.5 atm O2. The box shows some
permutations for reaching this threshold.

1.3 atm O2 = 100% O2 at 9.1 fsw (pure oxygen)

32%
O2 at 101 fsw (Nitrox)

21% O2 at 172 fsw (compressed air)

1.5 atm O2 = 100% O2 at 16.5 fsw (pure oxygen)

32% O2 at 122 fsw (Nitrox)

21% O2 at 203 fsw (compressed air)

1. When diving with compressed air, what is the percentage of inhaled oxygen at
each of the following depths?a. 33 fsw
b. 66 fsw
c. 99 fsw

2. When diving with compressed air, what is the atm. of inhaled oxygen at each of
the following depths?a. 33 fsw
b. 66 fsw
c. 99 fsw

3. A diver is treated at sea level, in an emergency room, with increasing
concentrations of oxygen (provided via loose-fitting face masks). For each concentration
of oxygen inhaled, state the amount in atm. O2.a. 40%
b. 60%
c. 90%

The risk of seizures from oxygen toxicity begins at 1.3 to 1.5 atm O2.
To reach this level on compressed air the diver has to exceed the RSD depth limits (see
box). Divers who go deep (technical or other) can reduce the risk of oxygen toxicity by decreasing
the concentration of inhaled oxygen. For example, a diver at 7 atm. (198 fsw) might switch
to a mixture containing just 4% oxygen (mixed with helium or helium and nitrogen). At sea
level, 4% oxygen would not support human life; at 7 atm., 4% oxygen is about the same as
breathing 28% oxygen at sea level. On the other hand, a diver breathing 21% oxygen at 7
atmospheres (198 fsw) would be at risk for oxygen toxicity as he would be inhaling 1.54
atm. O2.

Pure oxygen was used in re-breathing scuba equipment during World War II. Because of
the risk of oxygen toxicity, military divers were limited to about 25 fsw, or 1.76 atm. O2. (The military now uses mixed gases with its re-breathing scuba
apparatus for deeper diving). It is also because of oxygen toxicity that hyperbaric
treatment schedules limit the breathing of 100% oxygen to only about 20 minutes at a time.
In summary, the risk of oxygen toxicity is directly related to the total atm. of O2 activity. Examples of safe and unsafe oxygen concentrations are shown in
Table 1.

TABLE 1. Risk of Oxygen Toxicity with Supplemental Oxygen*

Atm. O2 Inhaled

Time before O2 toxicity may develop**

Depth equivalent in fsw breathing 100% O2

Depth equivalent in fsw breathing 21% O2 (compressed air)

0.21

never

0

0.42

indefinite

33

0.63

few hours

66

0.84

few hours

99

1.00

few hours

sea level

124

1.30

mins to hours

9.1

172

threshold***

1.50

mins to hours

16.5

203

2.00

minutes

33

281

3.00

minutes

66

438

4.00

seconds

99

596

fsw = feet of sea water

* For the resting or sedentary
individual; oxygen toxicity can develop more quickly with exercise

** Pulmonary or central nervous system effects

*** For risk of seizures

HOW DOES CARBON MONOXIDE TOXICITY OCCUR IN SCUBA DIVING?

Carbon monoxide (CO) is a tasteless, odorless, highly poisonous gas given off by
incomplete combustion of petroleum fuel. Virtually every gasoline powered motor, including
all cars that use hydrocarbon fuel, emit some carbon monoxide. All lighted cigarettes also
give off carbon monoxide. The extreme toxicity of CO arises from the fact that, compared
with oxygen, it combines about 200 times more readily with hemoglobin. As a result, any
excess CO readily displaces some oxygen from the blood; the more CO there is, the more
oxygen will be displaced.

CO-related problems while diving can occur two ways, one more infamous than the other.
Probably the less appreciated problem is simply from smoking. All smokers (cigarette,
cigar, pipe) have an elevated blood CO level and, sadly, many divers smoke (even on the
dive boat!). There is no evidence that diving increases the blood CO level in smokers, but
since CO competes with oxygen, the smoking diver is more hypoxic on entering and exiting
the water than otherwise. Any stressful situation thus puts the diver at increased risk
for an hypoxic-related event, such as heart attack.

While at depth, the hypoxic effect of excess CO will be somewhat (but not completely)
mitigated by the higher blood oxygen level that also occurs at depth. In final analysis,
we really don't know to what extent smoking causes problems in divers, but common sense
(and basic physiology) makes it a dumb practice to smoke and dive.

The toxicity mechanism we hear more about is when enough CO is in the tank air to act
as a life-threatening impurity. Fortunately this is a rare occurrence, but it happens, and
the result can be truly disastrous. According to news reports in April 1994, soon after a
German scuba diver's body was recovered off Key West, Florida,

"investigators suspected something unusual...analysis
[of air in the diver's tank] revealed
carbon monoxide nearly three times the level
considered acceptable."

The analysis reportedly showed 2500 parts per million of CO in the tank's air, an
extraordinary level. Non-smoking city dwellers inhale about 10 p.m. (Ten p.m. is
considered the maximal CO level permissible level in scuba tank air). Cigarette smokers
inhale between 30 and 60 p.m. of CO; this amount binds from 5 to 10 per cent of the blood
with CO, which means 5-10% of the smoker's blood is unable to carry oxygen. An inhaled CO
level of 2500 would tie up over 60% of the blood and make anyone fatally hypoxic.

It was speculated that this diver's tank air was contaminated from a faulty air
compressor. Air can certainly become impure when tank filling takes place near machine
exhaust; the exhaust fumes can be taken up and compressed along with the surrounding air.
At depth the pressure of any CO inhaled from a scuba tank is increased just like every
other inhaled gas. However, unlike any other gas likely to be in the tank, even small
amounts of CO can be harmful, because CO has a great affinity for hemoglobin and easily
displaces oxygen from the blood.

Depending on the concentration of CO in the tank and the depth at which it is inhaled,
the effects of CO toxicity may range from mild headache to confusion to a state of
unconsciousness and death. Any CO impurity must be considered potentially dangerous at
depth.

The incidence of faulty tank air is very rare, at least at reputable fill stations, so
it is impractical to do on-site chemical analysis of every tank. Until air analysis
becomes routine (if ever), testing must be up to the diver's senses, which means taking
several breaths from the tank before entering the water. This practice helps provide a
regulator check as well as a cursory check of the tank air. Certainly any headache (from
CO) or bad taste (from other impurities) is warning that something may be wrong with the
air. (Such a cursory check will likely not detect low levels of impurities, so sticking
with a reputable fill station is probably your best protection.)

WHAT IS CARBON DIOXIDE TOXICITY?

Carbon dioxide is a gas byproduct of metabolism. Our body makes about 200 cc's of CO2 every minute (more when we exercise) and excretes it in the air we
exhale. Plants take up the CO2 and give off oxygen (photosynthesis).
The concentration of carbon dioxide in the atmosphere is almost zero, and poses no risk
when fresh air is compressed inside a scuba tank.

The partial pressure of carbon dioxide in a scuba diver's blood is a function only of
metabolism and the rate and depth of breathing the same factors that determine blood CO2 concentration on land. Unlike other gases normally inhaled (nitrogen and
oxygen), or gases that could be inhaled under abnormal conditions (CO and other gas
impurities), the CO2 level in the blood is unchanged by the
ambient pressure (i.e., the depth) per se.

Scuba apparatus used in recreational diving is "open circuit," so exhalation
of carbon dioxide is through the mouthpiece and into the water (it's all in the bubbles).
Abnormal carbon dioxide accumulation in the blood can occur from too high a level of
metabolism (from exercise) and/or inadequate breathing (usually not breathing deep
enough). The medical term for high carbon dioxide in the blood is hypercapnia; when
the level is high enough it can cause "CO2 toxicity,"
which can lead to shortness of breath, headache, confusion and drowning (depending on
severity).

Air density increases with depth, so the deeper you go the greater the work of
breathing. Increased resistance to breathing can cause the diver to take shallow breaths,
and shallow breaths make carbon dioxide elimination less efficient. If the diver also
exerts herself heavily, her body will produce more CO2, resulting in
a "vicious cycle" of carbon dioxide buildup: heavy work (more CO2
production) ---> shallow breathing (less efficient elimination of CO2)
---> higher blood CO2 (CO2 toxicity).

Hypercapnia (and resulting CO2 toxicity) is a major concern among
deep divers, and also any diver who has to perform heavy work. It is much less of a
concern for the typical recreational diver. Regular, deep breathing, and a properly
functioning regulator, should eliminate risk of carbon dioxide buildup in recreational
diving.

Some experienced divers practice "skip" breathing, which is holding the
breath (on inhalation or exhalation) in order to conserve air. This might save air but it
could also lead to CO2 buildup, since by breath holding the diver
is, in effect, under ventilating; if the diver under ventilates he will soon want to
breathe even more, from the stimulus of an increasing CO2 level. As
a result, the diver who skip breathes enough to increase his CO2
could end up depleting air supply faster than with normal breathing! Even without the
obvious risk of pulmonary barotrauma (particularly if near the surface), skip breathing is
definitely not recommended.

CO2 BUILDUP WHEN DIVING: RISKS

Diving deep
Heavy work
Rapid, shallow breathing
"Skip" breathing

Apart from the practice of skip breathing, for recreational divers the depths achieved,
the short times spent on deeper dives, and the open circuit design of scuba equipment make
CO2 toxicity an uncommon problem. Just be aware that, under some
conditions, it can occur.

4. A diver breathing compressed air has a seizure after 10 minutes a 66 fsw. Until
the onset of seizure he is observed to have no breathing difficulty or any problem with
equipment. Rank the following potential causes of his problem from 1 (most likely cause)
to 5 (least likely).a. nitrogen narcosis
b. CO toxicity
c. CO2 toxicity
d. O2 toxicity
e. epilepsy (spontaneous seizures)

5. A diver breathing Nitrox I (32% oxygen, balance nitrogen) air has a seizure
after 20 minutes at 140 fsw. Until the onset of seizure he is observed by his buddy to
have no breathing difficulty or any equipment problem. Of the choices provided below, what
is the most likely cause? Can the other four conditions be ranked as in the previous
question?a. nitrogen narcosis
b. CO toxicity
c. CO2 toxicity
d. O2 toxicity
e. Epilepsy

6. A diver breathing compressed air develops a headache after 20 minutes at 90 fsw.
She is breathing fast and shallow and finds it difficult to inhale from her regulator.
Tank psi is 1500 psi. The most likely cause of her headache is:a. nitrogen narcosis
b. CO toxicity
c. CO2 toxicity
d. O2 toxicity

7. Of the choices a. through d. in question 6, which one is the least likely cause
of the diver's headache?

8. A diver is at 130 fsw. hovering next to a wall, when he is seen to suddenly
drift downward, seemingly oblivious to the increasing depth. His buddy catches up with him
at 145 fsw and pulls him up to 100 fsw. The most likely explanation for the sinking
diver's behavior is:a. nitrogen narcosis
b. CO toxicity
c. CO2 toxicity
d. O2 toxicity

Many divers mistakenly believe that any headache under water is due to carbon dioxide
buildup. For reasons discussed in the preceding section, CO2 buildup
is uncommon in recreational divers. Even with CO2 buildup, head-ache
may not be a symptom. In several studies the first symptom of CO2
buildup was sudden blackout, with no headache or other warning signs.

More importantly, there are many other (and more plausible) causes for a headache while
diving. The potential causes include: tank gas impurities (e.g., low levels of carbon
monoxide); temporomandibular (jaw) joint ache from holding the mouthpiece too tightly;
pressure of the mask against the forehead; a tight mask strap; salt water in the nasal
passages; tension or anxiety; cold water; over breathing (hyperventilation); and squeeze
on inadequately ventilated frontal sinuses.

In summary, headache is a sign that something is not right, but (in recreational
divers) it is not a sure sign of CO2 buildup.

WHAT IS SHALLOW-WATER BLACKOUT?

As has been pointed out, diving without compressed air (breath-hold diving, skin
diving) is very different from scuba diving, since the lungs compress on descent and
decompress on ascent. Water pressure squeezing the lungs during a breath-hold dive is
usually not great enough to cause problems from compression of the lungs (most breath-hold
divers don't go deep enough to experience significant lung squeeze). Middle ear discomfort
is a more common problem, and the breath-hold diver must swallow or blow against a pinched
nose to equalize ear pressures.

Since, in a breath-hold dive, air compressed on descent merely expands back to its
original volume on ascent, there is no danger of pulmonary barotrauma. But breath hold
diving is not without hazard. Perhaps the most serious potential hazard for the
breath-hold diver is "shallow-water blackout."

Shallow-water blackout is a sudden unconsciousness from lack of oxygen during a
breath-hold dive. (The term was originally applied, in the 1940s, to blackout from CO2 buildup seen with re-breathers; over the years the term's definition has
been changed.) Shallow-water blackout doesn't always occur in shallow water; it can occur
at any depth. However, for reasons which will be explained, the breath hold diver is at
greater risk for blacking out during ascent, near the surface.

To appreciate shallow-water blackout, consider the air hunger you feel during a
breath-hold dive. When you hold your breath two things happen in the blood; CO2 increases and O2 decreases. The principle reason
you feel air hunger is the increase in CO2, not the decrease
in O2. Without the slight increase in CO2 from
breath holding, your sensation of air hunger would be delayed and you would stay down
longer, even while your oxygen level is falling. This is an example where CO2
buildup is a good thing!

The risk of shallow-water blackout is increased from excessive over breathing
(hyperventilation) just prior to the dive. Hyperventilation lowers blood CO2.
At most one should take three to four deep breaths before a breath-hold dive; more than
that can lower CO2 sufficient to delay its buildup and therefore the
urge to breathe and surface for air. In other words, blood CO2 may
be lowered so much by pre-dive hyperventilation, that it takes a relatively long time for
CO2 to build up under water and cause "air hunger." The
dive is prolonged but at the diver's peril. Blood oxygen will fall relatively quickly
under water compared to the buildup of CO2. A critical hypoxic state
can be reached before there is any drive to breathe, i.e., before there is any sensation
of air hunger. This critical hypoxia is often reached on ascent, near the surface, hence
the term "shallow-water blackout." However, it can occur at any depth and lead
to sudden unconsciousness and drowning (Figure 1).

There is another factor that contributes to hypoxia, one that helps explain why
blackout tends to occur near the surface. Even though the body utilizes oxygen throughout
the breath hold dive, at depth the water pressure effectively increases oxygen pressure in
the lungs and the blood. All the while, of course, the diver is metabolizing oxygen, so
the total amount available is steadily declining. Paradoxically, however, being
deep is somewhat protective, because the pressure of oxygen in the lungs and blood is higher
than it would be at the surface with the same breath holding time. However, as the breath
hold diver rises toward the surface, the pressure of oxygen in his lungs falls
precipitously, not only because his body continues to utilize oxygen, but also because the
surrounding pressure falls. Near the surface the breath-hold diver's blood oxygen pressure
falls to a critical level and he blacks out. (Ambient pressure falls on ascent from a
scuba dive as well, but the oxygen supply is continuously replenished with fresh air from
the tank).

Figure 1. Example of changes in oxygen and carbon dioxide that can lead to
shallow-water blackout in a breath-hold diver. Direction of arrows indicates PO2 and PCO2 values above (up) or below (down) normal
land values. Initial changes show a diver who has hyperventilated just prior to the dive.
Actual point at which PO2 and PCO2 reverse,
and the degree of change, will depend on depth of dive, time under water, and work exerted
on the dive.

A CASE OF "SHALLOW-WATER BLACKOUT"

A young scuba instructor working on a liveaboard dive boat, and one of the boat's male
guests, decide to go breath-hold diving one afternoon. They, and two other boat guests
along just for the ride, take a dingy out to the site of a famous wreck.

Each of the breath-hold divers carries four pounds of lead weight to assist in descent,
and while one dives the other stays in the water as a spotter. While the non-diving guests
remain in the dingy, the divers each make a breath hold plunge. The first dive for each
lasts about 1.5 minutes, at a depth of 60 to 70 feet.

On the scuba instructor's second breath-hold dive, he goes a little deeper and stays on
the wreck a little longer. Over 2 minutes into his dive, he is seen to ascend quickly from
the wreck, then stop at 10 feet from the surface; at that point he shows no movement. The
spotter dives down and drags the unconscious diver to the surface. The rescuing diver
provides in-water mouth-to-mouth resuscitation and, with the aid of the two other people,
lifts the by now semi-conscious diver into the dingy. The rescued diver fully regains
consciousness but remembers nothing about what happened.

A few minutes later they are back on the liveaboard. The dingy observers reveal what an
awful sensation they felt as the limp instructor was pulled to the surface; they thought
he might be dead. The rescued scuba instructor only complains of having some chest
discomfort and feeling fatigued. He is also observed to have blue nail beds (cyanosis). He
is given 100% O2 and, when he claims to feel better, goes to lay
down in his cabin. A few hours later he feels worse and has a fever; the captain decides
to motor to the nearest town, where the diver is hospitalized. Diagnosis: pneumonia
(presumably from aspiration of some sea water.) He is given antibiotics and the next day
is released; he eventually recovers fully.

Youth, diving experience and excellent physical condition allowed the scuba instructor
to stay down much longer than the average person; this was also his (almost fatal)
undoing. What happened is that his delayed urge to breathe made him attempt an ascent too
late; just 10 feet from the surface he blacked out from lack of oxygen. Had there
not been an experienced spotter on the surface the instructor would surely have drowned.

Answers to TEST YOUR UNDERSTANDING

1. a, b, c. All are 21% Oxygen

2. a. .42 atm. O2
b. .63 atm, O2
c. .84 atm, O2

3. a. .40 atm, O2
b. .60 atm, O2
c. .90 atm. O2

4. 1st., epilepsy; 2nd., CO toxicity (due to contaminant in the tank air). These are
the only two plausible causes among the five listed. CO2 toxicity would be
unlikely due to the short time of the dive, the shallow depth, and the lack of a breathing
or equipment problem. Nitrogen narcosis would also be unlikely at this depth. Oxygen
toxicity would not occur at this depth after breathing compressed air for 10 minutes.

5. 1st. O2 toxicity. The other four causes are all possible but unlikely.
Epilepsy can cause seizures at any time; a "first-ever" seizure under water is
unlikely to be epilepsy, so the pre-dive medical history would be very important. CO2
toxicity usually does not present with seizures, but it can lower the threshold for
developing oxygen toxicity. Nitrogen narcosis also does not acause seizures. Finally, CO
toxicity is always a concern in sudeen and unexplained underwater castrophes, but it
occurs only with major contamination of tank air.